When training or packing for the challenge, it’s best to select loose-fitting items you can layer. Clothing can be removed or put back on as needed. And loose clothing that wicks away moisture permits free airflow and ventilation (so your favorite cotton, hooded sweatshirt may be best saved for the morning cup of tea or movie at home). Synthetic-blend materials wick away moisture and dry quickly when the weather is hot. During winter training, make certain your walking attire keeps you both warm and dry.
Rain gear should be breathable, lightweight, and certainly waterproof. There are several manufacturers that produce garments that meet these criteria.
The pros at REI have created a menu of articles on how to choose performance clothing.